The Electronic Molding Compound (EMC) industry has historically utilized various forms of silicon dioxide powder (silica, SiO.sub.2) as inert, inorganic fillers in thermosetting resin compositions for electronics encapsulation applications. The silica, while providing some degree of improved thermal conductivity and physical properties, is predominantly used to reduce the water adsorption of the composite compound, especially when used in very high solids concentration. The industry has many years of experience with silica, and thus most of the EMC compositions used are optimized to maximize the compatibility between the epoxy matrix and the SiO.sub.2 filler. Further, the low thermal expansion of silica is well suited to this application, as it provides a product that more closely matches the expansion of the silicon chips and thus reduce thermal stresses. In cases where thermal conductivity was an important design criteria, thermal management of components encapsulated with this material have historically been accommodated by incorporation of metal slugs within the compounded encapsulant, or by heat spreaders attached above the encapsulation material.
Continued need for reduced size in electronic packages is necessitating the removal of the metal heat spreaders and instead, removing heat by improving the thermal conductivity of the encapsulation material directly. Thus, the industry has been evaluating the use of thermally conductive fillers in EMC compositions, including aluminum oxide (alumina) and aluminum nitride. Alumina has already found some niche applications in this regard. However, its high hardness, coupled with its higher thermal expansion coefficient and moderate thermal conductivity makes it of limited value to the industry.
Aluminum nitride (AlN) is significantly more thermally conductive than either silica or alumina; providing a large potential improvement in this property in the compounded polymer. Additionally, AlN exhibits a relatively low thermal expansion coefficient, and is thus it provides for a better match with the silicon chips than alumina. Also, AlN is not as hard as alumina, and thus should reduce equipment wear during processing.
However, aluminum nitride in an as-synthesized form is not suitable for EMC applications due to its tendency to hydrolyze in contact with water or humid environments. The hydrolysis reaction results in the formation of aluminum oxide on the surface of the AlN particles, which reduces their thermal conductivity. It also forms ammonia gas, which can swell and eventually crack the polymer matrix. Thus, in order to use AlN in this application, the material must be made insensitive to moisture to fully protect against hydrolysis. In concept, AlN can be rendered water-stable by application of a coating to all particle surfaces. One such form, a silica coated AlN powder, has the added benefit of presenting a similar surface chemistry as the silica powders already used widely in the industry, thus requiring little change in existing formulations or processing from current practice.
Prevailing barriers to widespread utilization of AlN and coated AlN powders in the EMC industry include concerns over long-term hydrolytic stability, and mechanical stability of the coating. However, a growing need exists to resolve the issue of heat removal in electronics through the encapsulant, which is served by the thermal properties of AlN. Thus, methods of preparing coated AlN that consistently exhibit excellent hydrolysis stability are of keen interest.
A number of companies, including ART, have developed coated powders for applications requiring resistance to hydrolysis, such as for EMCs. Coating powders with silica to protect the powders from hydrolysis has been discussed in the literature for a number of powders, including, for instance, aluminum nitride and magnesium oxide. The physical characteristics determined for silica coated AlN powders in particular include coating thickness of 300-400 nm, oxygen contents in the 2-4% range, and surface area ranging from 8-12 m.sup.2 /g. These powders provide a substantial improvement in hydrolysis stability compared to uncoated AlN powders. However, materials designated for use in the EMC application must pass extremely rigorous tests, including those that aggressively investigate the hydrolysis stability of the material in application-relevant conditions. These hydrolysis tests include 85/85 testing, which involves exposing the material to a controlled environment at 85% relative humidity and 85.degree. C. and evaluating hydrolysis-related weight gain over 1000 hours; and the Highly Excellerated Stress Test (HAST) where samples are exposed to 100% humid conditions at high temperatures and pressures. In many cases, HAST testing is performed at 155.degree. C. and .about.60 psi, for periods of tens to hundreds of hours.
While silica coated AlN powders have exhibited significantly improved performance in these tests compared to uncoated AlN, further improvements in hydrolysis stability is of interest to the EMC industry. Further, while the silica coated AlN powder exhibits very good hydrolysis stability as prepared, it must withstand significant mechanical processing in the EMC application. This mechanical processing can damage the silica coatings on the particles and cause a reduction in stability of the material. Thus, it is important also to provide a coated AlN product that exhibits damage tolerance and retains high hydrolysis stability.
Significant prior art exists which describes processes and products related to silica coated inorganic powders. However, in all cases, the described processes concentrate on the procedures that cause the deposition of silica onto the inorganic particles, which in all cases involves nucleation and growth of the silica coating in a particulate mode in the vicinity of the surfaces of the inorganic particles. In some cases, thermal processing may follow the deposition process, but these thermal processes always involve moderately low temperatures, and typically are performed to facilitate low temperature reactions such as solvent evolution or phase modification. The typical process by which silica coatings are formed on inorganic particles involves the hydrolysis of silicon-based alkoxides such as tetraethylorthosilicatc in the presence of the core inorganic particles. The hydrolyzed silica forms fine particles with diameters on the order of nanometers, which then coat the inorganic cores by a deposition process, assisted by hydrogen-type bonding between the coating particles and the cores. The deposited silica particles can be assumed to maintain their size and shape due to the relatively low temperatures of the process, and thus the particles also retain their other physical attributes, including surface area.
The high surface area measured on coated powders relative to initial, uncoated powder is suggestive of the structure of the coating, and is critical to an understanding of the invention described in this patent. In the specific case of silica coated AlN, the coarse (uncoated) AlN powders used for these applications exhibit surface areas of approximately 1 m.sup.2 /g. In order for the overall surface area of the silica coated AlN powders to be .about.10 m.sup.2 /g, an extremely high surface area must be attributed to the to the coating. Given the small thickness of the coating layer, it can be shown by calculation that this increase in surface area cannot be due to any possible increased surface roughness related to the coating. Rather, it is expected that the coating consists of the very small, individual silica particles that precipitated during the coating process and subsequently bonded in the heat treating step. It is these fine particles themselves that are contributing so strongly to the surface area of the overall powder sample. In order for the overall powder to exhibit a 10 m.sup.2 /g surface area, the coating itself would need to be in excess of 200 m.sup.2 /g. If the silica particles are assumed to be spherical, their particle diameter would be .about.10 nanometers to yield this high surface area. Such fine particle sizes are not unexpected for inorganic particles such as silica prepared by hydrolysis of alkoxide materials.
Further, these fine particles can only influence the surface area of the bulk powder if their surfaces are exposed to the analysis technique. Surface area is typically measured by adsorbtion and desorbtion of nitrogen atoms on the intended samples--because nitrogen will adsorb to specific volumes on exposed surfaces, the desorbed nitrogen content can be correlated to the surface area of the sample. However, it must be noted that this process can only interrogate exposed porosity, as it requires the penetration, adsorbtion and desorbtion of the nitrogen on the surfaces during the course of the procedure. Thus, in the case of silica coated AlN particles, where the measured surface area is on the order of a magnitude higher than the original uncoated particles, such high surface areas can only come about if the silica coating is highly porous, i.e., that the silica particles are packed together, but not consolidated; such that the void regions between particles allows for interaction with the external environment. Thus, assuming the coating is constructed of bonded spherical particles of silica (rather than an impervious silica shell), it can be estimated from a very conservative view of monosized particle packing theory that the pore volume in the coating is greater than at least 30%, with mean pore diameter of .about.10 angstroms. (not to scale) is a representation of an AlN sphere about which is deposited a coating of SiO2 in the form of discrete spheres (drawing not to scale). Such a coating could be expected to form a cohesive layer and adhere to the particle, thereby provide some degree of water resistivity. Such fine porosity can be expected to represent a significant barrier to fluid water penetration, and even exchange of vaporous water with the AlN core. However, the permeable nature of the coating will eventually result in water penetration and hydrolysis of the AlN core. Should this penetration and subsequent reaction cause disruption of the surface coating, then water penetration and the resulting hydrolysis will increase in rate until the AlN core integrity has been compromised altogether.
When using such a coated powder for filled polymer applications, the ramifications of the coating structure are highly important. Most importantly, the fact that the coating is permeable suggests that it is not an absolute shield against hydrolysis of the underlying AlN. While the small average pore size and relatively low pore volume will significantly decrease water mobility, it can be expected that water penetration, particularly in the form of water vapor, will eventually occur, resulting in hydrolysis and slow decay of the AlN particles.
Hydrolysis testing involves direct exposure of the sample water-bearing environments, such as 85% relative humidity air at 85.degree. C. Weight gain due to these conditions is used to determine onset and extent of they hydrolysis reaction. The silica materials typically used in this industry routinely exhibit moisture-related weight gains on the order of 0.5-1.0% as no reaction occurs between the water and silica other than typical surface adsorbtion. FIG. 2 shows the degradation of a conventionally coated AlN powder (such as the product offered commercially by Dow Chemical under the trade name of SCAN.TM.) over a period of time at high temperature and humidity conditions. Weight gains above about 0.5% have been shown to be irreversible, which suggests a chemical reaction has occurred between the moisture and the core AlN. Note that weight gain greater than 0.5% is observed after about 350 hours of exposure, and ultimate weight gains are significantly above the nominal 1% level that is typical for silica fillers.
Additionally, it can be expected that the porous nature of the silica to coating will cause the coating to be mechanically weaker than a dense coating. The porosity in the coating can be expected to decrease the intra-coating strength as well as the coating-core particle bond. Thus, when handling such coated powders in commercial volumes and existing process machinery, the possibility of erosion, wear or fracture and spallation of the coating is real, and the result can be expected to yield an unacceptable coated product.